Continuously variable transmission

ABSTRACT

Continuously variable transmission provided with a primary pulley and a secondary pulley, around which a drive belt is arranged, clamped between two conical pulley discs of the respective pulley, a running surface of at least one pulley disc of the primary pulley and the secondary pulley, via which running surface this pulley disc contacts the drive belt, being provided, as seen in a cross section oriented perpendicular to a tangential direction, with a curvature, so that a pulley angle between a tangent on the running surface and a radial direction varies between a lowest value at the location of a radially innermost position on the running surface and a highest value at the location of a radially outermost position on the running surface. The curvature of the running surface of the primary pulley and that of the running surface of the secondary pulley differ from one another.

The present invention relates to a continuously variable transmission inaccordance with the preamble of Claim 1 for transmitting mechanicalpower between an engine and a load, it being possible for thetransmission ratio of the torque level and the rotational speed betweenthe pulleys to be varied continuously within a set range. Transmissionsof this type are generally known and are suitable in particular for usein motor vehicles.

As is known, in the transmission the drive belt is clamped between twoconical pulley discs of the respective pulleys, the ratio of the radialposition of the drive belt in the primary and secondary pulleys, whichis also known as the primary and secondary running radii, determiningthe transmission ratio of the transmission. The size of theabovementioned running radii can be varied in opposite directions byvirtue of the fact that one of the pulley discs of each pulley isarranged in such a manner that it can be moved in the axial direction.In the transmission ratio equal to one, the abovementioned running radiiof the drive belt are therefore equal to one another, while at atransmission ratio of greater than one the primary running radius isgreater than the secondary running radius, and vice versa.

It is known, inter alia from EP-A-0291129, that while the transmissionratio is being varied, the centre of the drive belt, i.e. a positionhalfway along the axial distance between the pulley discs, shifts in amanner which differs for each pulley. Consequently, the drive beltbetween the pulleys will, in virtually all possible transmission ratios,not be oriented perpendicular to the pulley axle, i.e. the axialdirection, but rather at a small and varying angle with respect thereto.This phenomenon is referred to as the skew running of the drive belt. Itis recognized that skew running of the drive belt may have adverseeffects on the functioning of the transmission, for example in terms ofincreased noise production or uneven wear. As a solution to thisphenomenon, it is proposed, inter alia in Japanese Patent Application63-053352, but also, for example, in JP-2002-031215, for the runningsurface of at least one pulley disc of each pulley, i.e. the contactsurface of this disc with the drive belt, as seen in a cross sectionoriented perpendicular to the tangential direction, to be provided witha curvature. By suitable selection of the said curvature, it is possibleto ensure that the drive belt always, i.e. irrespective of theinstantaneous transmission ratio of the transmission, advantageouslyremains oriented substantially perpendicular to the pulley axles.

In some cases, it is also possible to select to partially compensate forthe skew running by reducing at least the maximum angle of the drivebelt with respect to the axial direction. Moreover, as well as thecompensation for skew running, the literature also discloses otherphenomena for which the use of running surfaces which are curved to aprescribed extent may be advantageous. In each case, an angle between atangent on the respective contact surface and a radial direction, whichangle is referred to here as the pulley angle, runs from a lowest valueat the location of a radially innermost position on the contact surfaceto a highest value at the location of a radially outermost position onthe contact surface.

Although the known transmission fundamentally functions correctly,according to the invention it is still worthy of improvement. Inparticular, the invention relates to a reduction in the mechanical loadon the drive belt during operation of the transmission, in such a mannerthat its service life and/or maximum load-bearing capacity is improved.

To this end, the transmission according to the invention ischaracterized in accordance with Claim 1. Surprisingly, in atransmission of this type, the forces which occur between drive belt andpulleys during operation of the transmission are taken into account. Theinvention is based on the principle that an axially oriented clampingforce exerted on the drive belt by the pulley discs for the purpose oftorque transmission exerts a radially oriented force component on thedrive belt on account of the pulley angle between the pulley disc anddrive belt. On account of the fact that this force component is exertedon the two pulleys, the radial position or running radius of the drivebelt can remain constant, but at the expense of a tensile force in thedrive belt. This tensile force applies a mechanical load to the drivebelt, which is in principle undesirable, since this load makes little orno contribution to the transmission of torque between the pulleys. Theabovementioned force component and the tensile force in the drive beltresulting therefrom increase as the pulley angle becomes greater. Inview of this aspect, it may therefore be deemed advantageous to use thesmallest possible pulley angle.

However, a small pulley angle has the significant drawback that in theevent of a given deformation of the pulley discs, also known as theexpansion of the pulley discs, as occurs during operation under theinfluence of the clamping force, the drive belt coils inward to someextent in the radial direction between the pulley discs, away from theideal arcuate path contour. The extent to which this effect occurs isreferred to as the radial sag of the drive belt. When this occurs, partsof the drive belt slip with respect to the pulley discs, such that theefficiency of the transmission is adversely affected, which isfundamentally undesirable. Geometric considerations mean that theabovementioned radial sag and the resulting loss of efficiency increaseas the pulley angle becomes smaller, as the running radius of the drivebelt becomes greater or as the clamping force increases. In view of thisaspect, it may therefore be deemed advantageous to use the largestpossible pulley angle, certainly at a relatively high running radius.

The invention combines the abovementioned fundamentally contradictoryeffects in a surprising way, such that the service life and/orrobustness of the known transmission is enhanced while its efficiency isnot or is scarcely adversely affected. To this end, the invention ispartly based on the characteristic feature which arises as a result ofthe transmission being used in motor vehicles, whereby, duringoperation, in the lowest transmission ratio—as defined in accordancewith the present invention—it is subject to a relatively heavy but briefload, whereas in the highest transmission ratio it is subject to a lessheavy load but for a prolonged period of time. The lowest transmissionratio is used when a load, i.e. the motor vehicle, is being accelerated,for example from a stationary position, while the highest transmissionratio is used, for example, after the load has reached a desired, moreor less constant speed.

According to the invention, therefore, it can be concluded that it isadvantageous if, in the lowest transmission ratio, contact between thepulleys and the drive belt, both on the primary pulley and on thesecondary pulley, takes place at relatively low pulley angles, in orderto minimize the abovementioned tensile force, and if, in the highesttransmission ratio, the opposite applies, in order to minimize theabovementioned radial sag. Although the abovementioned curvature of therunning surface of the primary pulley does comply with an advantageousprofile of the pulley angle of this nature, according to the inventionthis is not the case for the secondary pulley. According to theinvention, it is advantageous if the highest value for the pulley angleof the secondary pulley is lower than that of the primary pulley. Afterall, it is this pulley angle which determines the contact between thedrive belt and the secondary pulley in the lowest transmission ratio, inwhich the tensile force effect prevails over the radial sag effect.

In order to leave the range in which the pulley angle of the secondarypulley varies, i.e. the difference between the highest and lowest valuesfor this angle, unchanged, the lowest value for the pulley angle, at asmall running radius, should be correspondingly lower. The expectationcould be that this would have a correspondingly adverse effect on theefficiency of the transmission in the highest transmission ratio.According to the invention, however, a disadvantageous effect of thisnature does not occur or scarcely occurs, since, as has already beennoted, the radial sag decreases proportionally to the decreasing runningradius, in an initial approximation even to the power of three. Near tothe smallest running radius, therefore, little or no radial sag occurs.

Nevertheless, according to the invention in a preferred embodiment ofthe transmission it may be advantageous for the lowest value of thepulley angle of the secondary pulley not to be correspondingly lower, sothat the range of the pulley angle for the secondary pulley is smallerthan the range for the primary pulley. This is because a lower limit forthe value of the pulley angle is generally necessary in order to realizea radially oriented clamping force component of sufficient magnitude toenable the friction between pulley and drive belt in the radialdirection to be overcome, so that the transmission ratio of thetransmission can be varied. In addition, in the case of a drive beltdesigned as what is known as a push belt with at least one continuousflexible ring, over the periphery of which a series of transverseelements, which can move freely over the ring at least in itslongitudinal direction, is arranged, sufficient tensile force has to berealized in the ring to enable the torque supplied to be transmittedbetween the pulleys by means of pushing forces between the transverseelements. This aspect is described in European Patent Publication EP-A 0931 959. In a particular embodiment of the transmission, the lowestvalue for the pulley angle of both pulleys is equal, for the abovereasons.

To maintain the effect which it is desired to achieve with a curvatureof the contact surface in accordance with the prior art, in which theprimary and secondary curvature are equal, despite the narrower range ofthe pulley angle of the secondary pulley, in practice it will often bedesirable to increase the range of the pulley angle of the primarypulley accordingly, for example by reducing its lowest value orincreasing its highest value. In the former case, according to theinvention the efficiency of the transmission will not or will scarcelybe adversely affected, since, as has already been indicated above, thediscs will scarcely expand in the vicinity of the lowest running radius.In the latter case, according to the invention it is even possible toexpect an improved action of the transmission, since the radial sagadvantageously decreases in the highest transmission ratio.

The invention is explained in more detail below with reference to thefigures and the exemplary embodiments illustrated therein.

FIG. 1 shows a diagrammatic cross section through a continuouslyvariable transmission of the known type.

FIG. 2 shows a pulley disc of the known transmission, and in particularits contact surface, in detail.

FIG. 3 shows a simplified side view of the transmission shown in FIG. 1.

FIG. 4 shows a graph plotting the pulley angle as a function of therunning radius of the drive belt for a primary pulley and a secondarypulley in accordance with the prior art.

FIG. 5 shows a graph corresponding to FIG. 4 in which the pulley anglesare determined in accordance with a possible embodiment of theinvention.

FIG. 1 diagrammatically depicts a cross section through the knowncontinuously variable transmission 1. The transmission 1 comprises aprimary pulley 2 which can be driven by an engine (not shown) and asecondary pulley 3 which drives a load (not shown), both of whichpulleys are provided with a pulley disc 21, 31 which is fixed to therespective pulley axle 20, 30 and with a pulley disc 22, 32 which can bedisplaced in the axial direction with respect to the said axle 20, 30. Adrive belt 10 is clamped between the pulley discs 21, 22, 31, 32, sothat mechanical power can be transmitted between the two axles 20 and 30with the aid of friction. The transmission ratio of the transmission 1is in this case determined by the ratio of a primary running radiusR_(P) and a secondary running radius R_(S) of the drive belt 10, i.e.its effective radial position between the pulley discs 21, 22, 31, 32 ofthe respective pulleys 10 and 20. The running radii R_(P) and R_(S) andtherefore the transmission ratio R_(P)/R_(S) of the transmission 1 canbe varied by causing the displaceable discs 22, 32 to move in oppositeaxial directions along the respective pulley axles 20, 30. In thefigure, the transmission 1 is illustrated by way of example with a hightransmission ratio, i.e. with a relatively large primary running radiusR_(P) and a relatively small secondary running radius R_(S).

FIG. 2 shows a more detailed illustration of an arbitrary pulley disc 43on the basis of a cross section seen in the tangential direction. Therunning surface 40 or contact surface 40 of the pulley disc 43 isprovided with a curvature, with a pulley angle α, defined between atangent 41 at a point R on the contact surface 40 and a radial direction42, increasing as seen in this radial direction.

FIG. 3 shows a side view of the known transmission 1, with the primarypulley 2 with the primary axle 20 on the left-hand side of the figureand the secondary pulley 3 with the secondary axle 30 on the right-handside of the figure. The lines L, M and H indicate the position of thedrive belt 10 in three transmission ratios of the transmission. Thedashed line L indicates the lowest transmission ratio, in which thetransmission 1, during normal operation, is subject to a relativelyshort but heavy load, for example when the load, i.e. the motor vehicle,is being accelerated from a stationary position. The dot-dashed line Hindicates the highest transmission ratio, in which the transmission 1,during normal operation, is subject to a relatively prolonged less heavyload, for example after the motor vehicle has reached a desired speed.The dot-dashed line M indicates, as an additional example, thetransmission ratio equal to 1, in which the primary running radius R_(P)is equal to the secondary running radius R_(S) and the primary axle 20and the secondary axle 30 have the same rotational speed.

FIG. 4 shows a graph which plots the pulley angle α—defined inaccordance with FIG. 2—of the primary pulley 2 and of the secondarypulley 3 in relation to the respective running radius R_(P), R_(S) ofthe drive belt 10. In this example, the pulley angles in relation to therunning radius α(R_(P)), α(R_(S)) are given by the known requirementthat the drive belt 10, in all possible transmission ratios R_(P)/R_(S)be oriented substantially perpendicular to the pulley axles 20 and 30.For both pulleys 2, 3 the pulley angle α varies between approximately7.5° and 10.8°.

FIG. 5 illustrates a similar graph, plotting the pulley angles inrelation to the respective running radius α(R_(P)), α(R_(S)), but inaccordance with a possible embodiment of the invention. According to theinvention, the highest value for the pulley angle of the secondarypulley (±8.8°) is advantageously smaller than that of the primary pulley(±11°), while the respective lowest values therefor (±7°) approximatelycorrespond to one another. Compared to the graph shown in FIG. 4, in thetransmission according to the invention the range for the pulley angle αfor the secondary pulley 3 has become smaller. Nevertheless, thetransmission according to the invention can also substantially complywith the requirement given above.

In addition to what has been described above, the invention also relatesto all the details shown in the figures, at least to the extent thatthey are directly and unambiguously recognizable to a person skilled inthe art, and to the features described in the set of claims whichfollows.

1. Continuously variable transmission (1) for motor vehicles,comprising: a primary pulley (2) and a secondary pulley (3), each of theprimary pulley and the secondary pulley comprised of two conical pulleydiscs (21, 22; 31, 32), a drive belt (10) arranged around the primarypulley and the second pulley and clamped between the two conical pulleydiscs (21, 22; 31, 32) of each respective pulley (2; 3), the drive beltin contact with a first running surface (40) of at least one pulley disc(43) of the pulley discs of the primary pulley (2) and with a secondrunning surface (40) of at least one pulley disc (43) of the pulleydiscs of the secondary pulley (3), the running surfaces each having, asseen in a cross section oriented perpendicular to a tangentialdirection, with a curvature, so that a pulley angle (α) between atangent (41) on the running surface (40) and a radial direction (42)varies between a lowest value at the location of a radially innermostposition on the running surface (40) and a highest value at the locationof a radially outermost position on the running surface (40), thecurvature of the first running surface (40) of the primary pulley (2)and the curvature of the second running surface (40) of the secondarypulley (3) differ from one another by the feature that the highest valuefor the pulley angle (α) of the secondary pulley (3) at a highestrunning radius (α(Rs)) is lower than the highest value for the pulleyangle (α) of the primary pulley (2) at the same highest running radius(α(Rp)), wherein a range between the highest value and the lowest valuefor the pulley angle (α) of the secondary pulley (3), over a range ofthe running radius of the secondary pulley, is smaller than acorresponding range of the pulley angle (α) of the primary pulley (2)over a corresponding range of the running radius of the primary pulley.2. Transmission (1) according to claim 1, wherein the lowest value forthe pulley angle (a) of the secondary pulley (3) is equal to the lowestvalue for the pulley angle (a) of the primary pulley (2) forcorresponding lower running radius of the primary and second pulleys. 3.Motor vehicle having an engine and a load which is to be driven, betweenwhich a transmission (1) according to claim 1 is incorporated, a powerwhich is to be generated by the engine being transmitted by the drivebelt (10) from the primary pulley (2) to the secondary pulley (3) andbeing released to the load by the secondary pulley (3).
 4. Motor vehiclehaving an engine and a load which is to be driven, between which atransmission (1) according to claim 2 is incorporated, a power which isto be generated by the engine being transmitted by the drive belt (10)from the primary pulley (2) to the secondary pulley (3) and beingreleased to the load by the secondary pulley (3).
 5. Transmission (1)according to claim 1, wherein, in relation to respective correspondingrunning radius (α(R_(P)), α(R_(S))) of the primary pulley and the secondpulley, the highest design value for the pulley angle (a) of thesecondary pulley (3) is ±8.8° and the highest design value for thepulley angle (a) of the primary pulley (2) is ±11°, and the lowestdesign value for the pulley angle (a) of the secondary pulley (3) andthe lowest value for the pulley angle (a) of the primary pulley (2) isequal to ±7°.
 6. Transmission (1) according to claim 1, wherein, inrelation to respective corresponding running radius (α(R_(P)), α(R_(S)))of the primary pulley and the second pulley, the value for the pulleyangle (a) of the secondary pulley (3) ranges from a first lowest valueto a highest value of ±8.8°, and the value for the pulley angle (a) ofthe primary pulley (2) ranges from the first lowest value to a highestvalue of ±11°.
 7. Transmission (1) according to claim 6, wherein, thelowest value for the pulley angle (a) of the primary pulley (2) and ofthe secondary pulley (3) is equal to ±7°.
 8. Transmission (1) accordingto claim 1, wherein, in relation to respective corresponding runningradius (α(R_(P)), α(R_(S))) of the primary pulley and the second pulley,an overall range of the values for the pulley angle (a) of the secondarypulley (3) is smaller than an overall range of the values for the pulleyangle (a) of the primary pulley (2).
 9. Transmission (1) according toclaim 8, wherein, the lowest value for the pulley angle (a) of theprimary pulley (2) is equal to the lowest value for the pulley angle (a)of the secondary pulley (3).